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Abstract

An alternative indium-free material for transparent conducting oxides of fluorine-doped
tin oxide [FTO] thin films deposited on polyethylene terephthalate [PET] was prepared
by electron cyclotron resonance - metal organic chemical vapor deposition [ECR-MOCVD].
One of the essential issues regarding metal oxide film deposition is the sheet resistance
uniformity of the film. Variations in process parameters, in this case, working and
bubbler pressures of ECR-MOCVD, can lead to a change in resistance uniformity. Both
the optical transmittance and electrical resistance uniformity of FTO film-coated
PET were investigated. The result shows that sheet resistance uniformity and the transmittance
of the film are affected significantly by the changes in bubbler pressure but are
less influenced by the working pressure of the ECR-MOCVD system.

Keywords:

Introduction

Transparent conducting oxide [TCO] has been used in many applications, such as solar
cells, flat panel displays, and smart windows [1]. Indium tin oxide [ITO] is used most widely for TCOs, owing to its high electrical
conductivity and transparency properties. On the other hand, the indium price has
soared recently due to the rapid demand of ITO [2]. To replace indium, many materials have been proposed for TCO of which one of them
is fluorine-doped tin oxide [FTO].

Sheet resistance uniformity of thin films deposited on TCO substrates is one of the
key successful parameters of the film quality. Previous studies examined ways of achieving
a high uniformity of deposit using a range of different deposition techniques, such
as pulse laser deposition [3], chemical beam coating [4], vacuum arc deposition [5], and plasma enhanced chemical vapor deposition. In addition, some studies also simulated
the uniformity models of the deposit through a Monte Carlo simulation [6,7]. All of them used glass as substrate, but it has been suggested that the deposition
of thin films on plastic substrates, such as polyethylene terephthalate [PET], is
still lacking due to limitations of the polymer physical properties, which requires
a low-temperature deposition [8].

Compared to glass, PET has its advantages because it is flexible and elastic, which
can make PET be shaped for many applications. One of the deposition methods that can
allow low-temperature deposition is electron cyclotron resonance - metal organic chemical
vapor deposition [ECR-MOCVD]. ECR-MOCVD can deposit a thin film at room temperature,
which will not deform a plastic substrate.

Depending on the techniques for growing the thin film, the process parameters such
as working and bubbler pressures significantly influence the quality of the thin film
[1,3,9]. This study deeply examines the effects of those parameters in ECR-MOCVD on sheet
resistance uniformity and transmittance of an FTO thin film coated on a PET substrate.

Experimental method

The ECR system consists mainly of two separate regions, i.e., the plasma source and
the deposition zone [10]. Cyclotron resonance is achieved when the frequency of the alternating electric field
is made to match the natural frequency of the electrons orbiting the lines of force.
This phenomenon occurs when the microwave source with a frequency and maximum power
of 2.45 GHz and 1,400 W, respectively, is introduced to the plasma chamber through
a rectangular waveguide under a static magnetic flux density of 875 G. A turbo molecular
pump was used to produce a vacuum. A 16 × 34 cm2 PET substrate was treated under a range of experimental conditions, as shown in Table
1. The plasma-assisted deposition lasted for about 15 min. Tetra methyl tin [TMT] and
sulfur hexafluoride [SF6] were used as the tin and fluorine precursors, respectively. The flow rates of gasses
were controlled by mass flow rate controllers, and TMT and SF6 were introduced to the chamber by argon gas as the carrier. Figure 1 shows the schematic diagram of ECR-MOCVD.

Figure 1.A schematic of ECR-MOCVD. ECR-MOCVD consists of an electromagnetic generator, microwave power source, and
plasma chamber, which is supported by vacuum-pump equipment.

A four-point probe was used to measure the sheet resistance at 16 points that are
fairly distributed on the samples. To observe the optical properties, a UV visible
spectrophotometer was used to quantify the transmittance of the FTO thin film deposited
on PET.

Results and discussion

The process parameters examined in this study are the working and bubbler pressure
variations. The working pressure is defined as the pressure applied in the plasma
chamber of ECR-MOCVD, whereas the bubbler pressure is described as the pressure introduced
to the TMT. Each of these parameters was examined by five different experiments with
the conditions as listed in Table 1.

Sheet resistance and transmittance of FTO film

A higher transmittance and a lower sheet resistance of the thin film are desirable
for TCO material applications. On the other hand, there is a trade-off among the two
parameters. Generally, a lower sheet resistance will be obtained when the thin film
is thick enough, which causes degeneracy on the transmittance.

Both parameters are correlated as the figure of merit [9], which represents the TCO performance as follows:

(1)

where T is the total visible transmittance at λ = 550 nm (%) and Rs is the sheet resistance (Ω/sq).

where UNI (%) is the sheet resistance uniformity; RsMax, RsMin, and RsAverage, ρ, and t are highest, lowest, and average of measured sheet resistance (ohm/square), resistivity
(ohm.cm), and film thickness (cm), respectively. Equation 2 indicated that the smaller
UNI (%) is preferable. Meanwhile, the higher UNI (%) represents a higher ranging value
between RsMax and RsMin which means that the film has a poor sheet resistance uniformity.

Because other process parameters of the deposition are fixed, the film thickness growth
under our ECR-MOCVD system is solely controlled by the bubbler pressure, as explained
in a previous work [1]. The prepared film thickness is ranging from 300 to 450 nm in our experimental range.

Effects of bubbler pressure variation

Figures 2a to 2e show the contour of the sheet resistance uniformity of each bubbler pressure variation
of 40.2, 43.3, 45.7, 55, and 60.6 Torr, respectively. The contour maps illustrate
the regions where the sheet resistances of each bubbler pressure variation are mapped.
The maps show the uniformity patterns that are formed in parallel with the y-axis obviously due to the application of a rolling system as a substrate holder in
the ECR-MOCVD apparatus. The sheet resistance uniformity is likely to be present at
the center of the coated PET substrate indicated by the larger areas with the same
color. These phenomena were also confirmed by previous researchers [11].

The evolution of fluorine [F-] atoms to tin oxide films has contributed to the substitution of oxygen O2- anions in the lattice and produces more free electrons, resulting in a decrease in
sheet resistance [9]. On the other hand, in the present study, the gas flow rate of the fluorine precursor
(SF6) and oxygen entering the plasma chamber were fixed at 12.46 sccm and 36.4 sccm, respectively.
Therefore, there were no influences from the fluorine concentration to the sheet resistances
of the thin film. As shown in Figure 2f, under bubbler pressure variations, the sheet resistance uniformity tends to increase
with increasing bubbler pressure. The sheet resistance was dramatically increased
when the pressure introduced was > 45.7 Torr. The rapid increase in sheet resistance
can be explained by the chemical kinetics, where a higher bubbler pressure results
in a higher flow rate of the TMT/Ar gas and increases the concentration of tin atoms
in the plasma chamber. The maximum sheet resistance was 315,600 Ω/sq when the bubble
pressure was 60 Torr after 15 min of deposition. Besides increasing the sheet resistance
of the film, the increasing bubbler pressure also penetrates the nonuniformity of
deposition. This can be observed as a large parity between the maximum and minimum
sheet resistances.

In terms of the optical properties, we found that the increasing bubbler pressure
affected the slightly higher film transmittance as shown in Figure 3a. At a wavelength of 550 nm, the transmittances of 40.2 and 60 Torr were 82.85% and
91.26%, respectively while the other transmittances of bubbler pressure variations
were in between. This situation follows the figure of merits as described in Equation
1; hence, one has to compromise the two parameters in order to obtain the appropriate
TCO materials.

Figure 3.The optical transmittance of an FTO thin film coated on a PET substrate. Optical transmittance for (a) bubbler pressure variations and (b) working pressure variations.

Influences of working pressure variation

Figure 4a to 4e show the contour maps for the experiments examining the working pressure variations
ranging from 5 to 15 mTorr. The maps suggest that the changes in working pressure
did not affect the sheet resistance of the thin film significantly. The working pressure
of 10 mTorr contributed to the highest average sheet resistance and was identified
as having a larger sheet resistance uniformity area in the center of the sample. In
the same manner as the sheet resistance, the transmittance of the FTO film coated
on PET was not really dependent on the changes in the working pressure of the apparatus;
in this case, the transmittance was similar (approximately 90%) as shown in Figure
3b.

Figure 4.Contour maps of an FTO thin film on a PET substrate for working pressure variations. The working pressures were varied at (a) 5 mTorr, (b) 7.5 mTorr, (c) 10 mTorr, (d) 12.5 mTorr, and (e) 15 mTorr. (f) The sheet resistance uniformity and figure of merit of the FTO thin film under working
pressure variations.

Conclusion

The effects of process parameter variations, in this case, the bubbler and working
pressures, on sheet resistance uniformity and optical transmittance of FTO deposition
on a PET substrate prepared by ECR-MOCVD were investigated. We found that the sheet
resistance uniformity and transmittance of the film were significantly affected by
the changes in bubbler pressure but were less influenced by the working pressure of
the ECR-MOCVD system. The sheet resistance uniformity tends to increase with increasing
bubbler pressure. The measurements carried out using a four-point probe showed that
bubbler pressures > 45.7 Torr cause a dramatic increase in the sheet resistance of
the film. The maximum sheet resistance was 315,600 Ω/sq at a bubbler pressure of 60
Torr after 15 min of deposition. At this point, the transmittance was excellent, i.e.,
91.26%. This could be understood as the figure of merit of the TCO materials where
both the sheet resistance and transmittance of the films can be compromised. For working
pressure variations, a pressure of 10 mTorr contributed to the highest average sheet
resistance and had a larger sheet resistance uniformity area in the center of the
sample. Overall, suitable process parameters are needed to obtain the desired sheet
resistance and transmittance of FTO films deposited on a PET substrate.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

All the authors contributed to the writing of the manuscript. JHP carried out the
experiments under the command of JKL. All authors read and approved the final manuscript.

Acknowledgements

This work was supported by the National Research Foundation of Korea Grant funded
by the Korean Government (MEST) under contract number NRF-2010-C1AAA001-2010-0028958.